1. Technical Field
The present invention relates to a burner for producing glass fine particles used for manufacturing optical fiber glass base materials (hereinafter, simply referred to as a “burner”) and to a method for manufacturing porous glass base materials using the burner.
2. Description of the Related Art
A variety of methods have been conventionally developed to manufacture optical fiber base materials. One of such conventional methods is the Outside Vapor Deposition (OVD) method that attaches and deposits glass fine particles generated in a flame of a burner onto a starting member having a thickness of approximately 50 mmφ while the starting member is being rotated and reciprocated relative to the burner, to obtain a soot having a thickness of approximately 250 mmφ to 350 mmφ, and then dehydrates and sinters the soot in an electric furnace. The OVD method is widely used due to its capability of producing optical fiber base materials having relatively desired refractive index distribution and mass-producing large-diameter optical fiber base materials.
In the conventional art, a concentric multi-tube burner is used to fabricate a deposit of glass fine particles. The concentric multi-tube burner does not sufficiently blend a glass material gas, a combustible gas, and an oxidizing gas. Therefore, no sufficient amount of glass fine particles are produced. As a result, the concentric multi-tube burner cannot achieve a high yield and has difficulties in fabricating deposits of glass fine particles within a short period of time.
To solve this problem, Examined Japanese Patent Publication No. 03-009047 proposes a multi-nozzle burner that is configured such that small-diameter oxidizing gas discharge ports (hereinafter, simply referred to as the “small-diameter gas discharge ports) are disposed within a combustible gas discharge port so as to surround a centrally arranged material gas discharge port.
Generally speaking, the small-diameter gas discharge ports are provided by several to several dozen small-diameter tubes that branch from a main tube connected to the gas inlet of the burner and thus relatively heavy. Therefore, the tubes become flexible due to their own weights in the high-temperature environment, which results in eccentricity at the end of the burner. To address this issue, Japanese Patent Application Publication No. 10-095623 proposes a technique of fastening the small-diameter gas discharge ports to adjacent inner or outer tubes in order to prevent flexure and vibration.
The multi-nozzle burner disclosed in Japanese Patent Application Publication No. 10-095623, however, includes a large number of small-diameter tubes, which are thus fastened in a very complicated fashion.
Because of the complicated fastening, such multi-nozzle burners are fabricated manually one by one. Therefore, large individual differences are observed among such burners. Specifically speaking, some burners suffer from a higher likelihood of attachment of glass fine particles onto the ends of the small-diameter gas discharge ports than others, and the deposition efficiency varies among the burners.